(1) Field of the Invention
This invention relates to a device for measuring rectilinear motions, which is useful, for example, for inspecting the accuracy of rectilinear motions of machine tools, optical devices, measuring devices and the like.
(2) Description of the Prior Art
Generally, the devices or apparatus which make rectilinear motions are provided with a movable table, for example, an X-axis table, X-Y-Z-axis table or the like. Therefore, in order to achieve an intended objective, it is a prerequisite of utmost importance to measure the accuracy of rectilinear motions of the device or apparatus with high precision.
In this connection, it is known that, when a rectilinearly moving body M like a machine tool is displaced along the Z-axis as illustrated in FIG. 12, there occur errors of six components, namely, a displacement error consisting of three components including a positioning error ez in the direction of the Z-axis, an error ex entailing the movement in the direction of X-axis and an error ey entailing the movement in the direction of the Y-axis, and an angular error consisting of three components including a pitching error .alpha. due to pitching about the X-axis, a yawing error .beta. due to yawing about the Y-axis and a rolling error .gamma. due to rolling about the Z-axis.
In this connection, we are aware of the following prior art literatures proposing rectilinear motion measuring devices which can simultaneously measure four of the above-mentioned six components, namely, the displacement error ex in the direction of the X-axis, the displacement error ey in the direction of the Y-axis, the pitching error .alpha. and the yawing error .beta..
(a) "Precision Measurement of Rectilinear Motions by the Use of Laser Beam" by Takada et al, pp. 123-124, Scientific Lectures at 1984 Autumn Meeting of Precision Machinery Society.
(b) "Advancihg Inspection of Accuracy of Rectilinear Motions at Machining Center" compiled by The Foundation for Promotion of Machine Tool Technology, May 1985.
In these prior art devices, a first polarizing beam splitter, a corner cube prism and a reflecting mirror are provided on the side of the rectilinearly moving body while providing coaxially a second polarizing beam splitter, a half mirror and a collimator lens on the side of the measuring device, and providing on the half mirror a laser beam generator for generating linearly polarized light and a quarter wavelength plate.
With such an arrangement, the luminous flux from the light source is passed through the quarter wavelength plate, half mirror and collimator lens and then split into a horizontally polarized component P and a vertically polarized component S by the first corner cube prism. By the corner cube prism, the vertically polarized component S forms reflected waves with a displacement of 2.DELTA.S which is twice the displacement .DELTA.S of the prism. The reflected waves are passed through the first polarizing beam splitter, collimator lens with a focal length f and half mirror, and, after being reflected by the second polarizing beam splitter, received by a 4-quadrant photosensor for detection of the displacement. The output signal of the photosensor is calculated as the X-axis and Y-axis displacement errors ex and ey.
Further, the horizontally polarized component P from the first polarizing beam splitter forms reflected waves with an angle of inclination which is two times as large as the angle of inclination .theta. of the reflecting mirror. The reflected waves are passed through the second polarizing beam splitter via the first corner cube prism, collimator lens and half mirror, and received by an angle detecting 4-quadrant photosensor as a displacement d.apprxeq.2.theta.f. The output signal of this photosensor is used to calculate the pitching error .alpha.and the yawing error .beta..
The above-described prior art devices have a number of inherent problems. Firstly, due to the large quantity of heat which occurs during the laser emission by the laser generator, the laser tube itself is thermally deformed with fluctuations in its laser output. Nevertheless, no measure is provided to prevent such fluctuations of the light source itself, which would impair the accuracy of the measurement.
Secondly, the reflected waves from the corner cube prism and the reflected waves from the reflecting mirror are passed along the same optical path, while transmitted through the collimator lens. Therefore, even if the corner cube lens is used to magnify the displacement .DELTA.S of the rectilinearly moving body to a doubled scale 2.DELTA.S, the magnified displacement is minimized as the reflected waves are transmitted through the collimator lens, resulting in low resolution of the displacement by the 4-quadrant photosensor.
Thirdly, of the six error components which occur to the rectilinearly moving body, it has been impossible to measure the rolling error Y when it is necessitated to measure same simultaneously with the above-mentioned four components.